Method of preparing molybdenum disulfide powder



This invention broadly releates to a method of preparing molybdenumdisulfide powder, and more particularly to an improved method forpreparing an exceedingly fine sized molybdenum disulfide powder havingthe particles thereof coated with a controlled quantity of an oilysubstance.

The exposure of substantially dry and oil-free molybdenum disulfidepowders. to the atmosphere causes oxidation of the particles formingoxides of molybdenum and sulfur which impart objectionable acidity tothe powder. The susceptibility of substantially dry and oil-freemolybdenum disulfide powder to oxidation increases as the particle sizethereof decreases exposing a greater surface area to oxidation attack.The oxidation of molybdenum disulfide powder is accompanied by anincrease in acidity which constitutes an objectionable characteristicparticularly when the powder is to be used as a lubricant and in whichuse the powder is preferably for an extremely fine particle size.

It is now known that coating the surfaces of the particles of amolybdenum disulfide powder with an oily substance will inhibitoxidation of the powder for extended periods of time. Oil coatings inconcentrations as low as 0.05% have been found to provide minimalprotection against oxidation of molybdenum disulfide powders. Since theaddition of a small amount of oil or oily molybdenum disulfide powder toa substantially oil-free molybdenum disulfide powder will not provide asatisfactory protective coating to the surfaces of all the particles ofthe substantially oil-free powder even though subjected to conventionalmixing techniques, relatively elaborate and expensive methods have beenheretofore employed to apply a substantially uniform coating of an oilysubstance on the surfaces of the powder particles. One such complexmethod is disclosed in United States Patent N0. 2,686,156 wherein an oilcoating is applied to an oil-free powder by a solvent containing a lowpercentage of oil and thereafter subjecting the powder to a controlledsolvent removal process.

It is a primary object of the present invention to provide an improvedmethod of coating substantially oil-free particles of a molybdenumdisulfide powder with an oily substance in a controlled amount so as toinhibit the oxidation of the powder and which method is simpler and moreeconomical than methods heretofore known.

Another object of this invention is to provide an improved method ofproducing an extremely fine oil coated molybdenum disulfide powdercontaining a controlled quantity of an oily substance and which methodconcurrently provides for substantially uniformly coating substantiallyoil-free particles with an oily substance while simultaneously reducingthe powder to a desired degree of fineness.

The foregoing and other objects of this invention are achieved byblending controlled proportions of a substantially oil-free molybdenumdisulfide powder with an tet oil-containing molybdenum disulfide powdercontaining a known oil content ranging up to about 8% oil and chargingthe blended powdered mass to a fluid energy impact pulverization milland pulverizing the blended powdered mass under controlled conditions toproduce a substantially dry and exceedingly fine sized molybdenumdisulfide powder having a substantially uniform coating of an oilysubstance on the particles thereof.

Other objects and advantages of the unique method comprising the presentinvention will become apparent from the following detailed description.

In the past, commercial molybdenums disulfide has been produced bygrinding molybdenite ore which is comprised largely of granite andmolybdenum disulfide and thereafter extracting the molybdenum disulfidefrom the gangue by an oil flotation extraction process. The amount ofgangue in the molybdenum disulfide powder is reduced to the desiredlevel by subjecting the ore to successive grinding and floatationoperations. The gangue which is comprised predominantly of silica and ishereinafter referred to as such as usually identified as the portioninsoluble in nitric acid and soluble in hydrofiuoric acid. Commercialmilling and flotation extraction of molybdenum disulfide frommolybdenite ore are usually continued until the silica content of theextracted molybdenum disulfide powder is reduced to a level below about12%. However, when a higher degree of purity is desired, such as, forexample, when the molybdenum disulfide powder is to be employed forlubrication purposes it is possible to reduce the silica content to alevel ranging from about .3% to about .5 by successive grinding and oilflotation extractions dur. ing which the extracted molybdenum disulfidepowder is reduced to an average particle size ranging from about Furtherreduction of the an elevated temperature such as, for example, atempera-.

ture of about 1200 F. to evaporate and thus remove the oil and and waterto contents of less than 0.03% oil and 0.03% water. Alternatively, ithas been proposed to subject the wet oily powdered molybdenum disulfidemass to a controlled retorting operation wherein substantially all ofthe water is. removed and the oilysubstance is reduced toa desiredlevel. Although the controlled retorting operation produces an oilcoated powder which is resistant to oxidation the method requiresrelatively expensive retort and retort controlling equipment to controlthe residuary oil content to within the desired range.

The molybdenum disulfide powders derived from the aforementioneduncontrolled retorting operation containing less than about 0.03% oil,or other molybdenum disulfide powders which are substantially oil-freeand derived from alternate molybdenite ore refining processes or othersources, are highly susceptible to oxidation when exposed to theatmosphere. tice of the present invention, the foregoing substantiallyoil-free powders can be blended in appropriate proportions with anoil-containing molybdenum disulfide powder of known oil content andthereafter pulverized in a fluid In, accordance. with the prac-- energyimpact pulverization mill whereby pulverization and coating of theoil-free particles is achieved. Oil containing molybdenum disulfidepowders suitable for this purpose include the wet oily powdered massderived directly from the oil flotation extraction refining ofmolybdenite ore and which contain up to about 8% flotation oils and upto about 16% water. The residuary flotation oil in the extracted powdercan be comprised of a wide variety of oily substances of a vegetable orpetroleum origin and mixtures thereof which will wet the surfaces of theparticles. Suitable oils for this purpose include refined petroleumoils, kerosene, pine tar oil, or any of the oily substances disclosed inUnited States Patent No. 2,686,156. It is beneficial under someconditions that the residuary oil content include some fairly highboiling point oils such as, for example, oils having an end boilingpoint of about 250 C. to facilitate control of the amount of oil removedby evaporation during the impact pulverization of the powder blend.

Suitable oil containing molybdenum disulfide powders also includeflotation extracted powders which have been subjected to an interveningchemical treatment to reduce the silica content thereof withoutmaterially reducing the oil content of the powder. It is alsocontemplated that the wet oily molybdenum disulfide powder, whether ornot subjected to the prior chemical treatment, can alternatively betreated in a preliminary drying step wherein all or a substantialportion of the moisture content is removed. The drying operation may beachieved by any one of a number of well known methods to evaporate thewater without significantly reducing the oil content of the powder. Ineither case, either the wet silica free or substantially dry oilcontaining molybdenum disulfide powder of known oil content ispreliminarily blended with the substantially oil-free powder inconventional mixing equipment to achieve a substantially uniform mixturethereof.

The appropriate portions of oil-free powder to oil containing powder ofknown oil content can be calculated so that the resultant blend willcontain the desired residuary oil content. As heretofore mentioned, acoating of oil in an amount as low as .05% will provide minimalprotection of the molybdenum disulfide particles against oxidation.Protection against oxidation increases as the oil content in the powderincreases. Seemingly dry free-flowing molybdenum disulfide powdershaving an oil coating on the particles thereof can contain from .05 upto about .5% oil. These free-flowing powders are particularly desirablefor use as solid lubricants. Oil contents above about .5 impair thefree-flowing characteristics of the molybdenum disulfide powder but aresatisfactory for compounding the powder with suitable lubricating oilsand greases. Depending on the specific use of molybdenum disulfidepowder, the proportions of the substantially oilfree and oil-containingpowders in the blend can be controlled so as to produce a pulverizedproduct containing from .05 oil up to a level approaching the oilcontent of the oil-containing powder, such as, up to about 8%. In viewof the fact that a reduction in oil content usually occurs during thefluid energy impact pulverization of the powdered blend, depending onthe milling conditions as will be subsequently described in detail, itis preferred that the proportions of oil-free and oil containingmolybdenum disulfide powders be controlled so that the powder blendchargedtothe mill contains a quantity of oil surficient to assure thatthe resultant pulverized product contains residual oils, for example, inthe range of from about 0.15% to about 0.3%, thus providing better thanminimal protection against oxidation and still assure a free-flowingpowder.

Pulverization of the molybdenum disulfide powder is achieved in animpact pulverization mill in which the powder is reduced in particlesize through the action of impact and abrasion of the individualparticles against each other at high velocities while entrained in astream of circulating gaseous fluid. The gaseous fluid can be anycompressed gas or vapor such as, for example, air, carbon dioxide,nitrogen, superheated steam, or a partially inert gaseous fluid such asresults from the evaporation of a portion of the residual oils duringimpact pulverization of the powder at elevated temperatures. Impactpulverization mills suitable for this purpose are made by Fluid EnergyProcessing and Equipment Company of Philadelphia, Pennsylvania, and bySturdevant Mill Company of Boston, Massachusetts. Impact pulverization,or mi cronization as it is usually referred to, can be controlled toachieve various particle size distributions. The heavier or largerparticles are retained in the mill while the finer or desired sizedparticles are carried out in the fluid stream. The particle size of themolybdenum disulfide powder blend charged to the impact pulverizationmill is usually in the order of an average particle size ranging fromabout 10 to about 250 microns and is thereafter materially reduced bymicronization to an average particle size such as, for example, 5'microns or less. Oil coated molybdenum disulfide powders having anaverage particle size in the order of about 5 microns or less, areparticularly desirable for dry lubricants or when compounding thepowders with suitable lubricating oils and greases.

During the milling operation of the powder blend comprising oil-free andoil-containing molybdenum disulfide powders, the impact and abrasionbetween the oil coated particles and oil-free particles is eflective totransfer a portion of the oil to the oil-free particles, substantiallyuniformly coating the surfaces thereof in addition to coating thefreshly exposed surface area created by the breakdown of the particlesinto smaller sizes. The oil coating mechanism, while not exactlyunderstood, occurs quickly thereby inhibiting oxidation of the powderduring impact pulverization. It is believed however, that at the pointof impact between the particles, suficient residual oil is evaporatedand condenses on the immediately adjacent newly formed oil-free surface.The effectiveness of this coating mechanism is surprising, in view ofthe fact that the direct coating of molybdenum disulfide powders withsuch small quantities of oil by ordinary mixing techniques is virtuallyimpossible and moreover, that the surface area of the powder isincreased many times during impact pulverization.

In addition to applying a substantially uniform oil coating to theoil-free particles and comminuting the powder blend to the desireddegree of fineness, the volume and temperature of the gaseous fluid, aswell as the feed rate of the powder blend to the mill can be controlledto achieve a controlled reduction of the water content of the feedmixture. Rapid and efficient drying of a powder blend containing up toabout 16% water can be achieved during the impact pulverization millingby using heated gases such as compressed air, for example, attemperatures ranging from room temperature (about 65 F.) to about 700 F.As the temperature of the gaseous fluid is increased, and the volumethereof is increased relative to a fixed weight of feed material, theefliciency of drying is increased. Accordingly, these variables can becontrolled to achieve any desired drying effect. The impactpulverization of wet and oily molybdenum disulfide powder blends isordinarily effective to reduce the water content to a level below about.5 Water contents of about .5 in the pulverized product are notobjectionable for most uses. However, it is preferred that the millingoperation be conducted under controlled conditions so as to removesubstantially all of the moisture or to subject the wet oilcontainingpowder prior to blending to a preliminary drying step as hereinbeforedescribed. Since impact pulverization conditions which are conducive toremove the water from the powder blend feed material are also conduciveto removing a portion of the residuary oil content, it is necessary thatthe oil content of the powder feed material be of a level so as tocompensate for any loss that may occur. In addition to the volume andtemperature of the gaseous fluid and the rate of feed of the powderedblend to the impact pulverization mill, the quantity of oil removedduring the milling operation is also dependent on the particularvolatility characteristics and the quantity initially present. Theamount of oil removed for any given powder blend feed material duringthe milling operation at specified conditions can readily be establishedby trial. As higher temperatures are required to control the oil contentat the desired level, the tendency to oxidize the particles alsoincreases when an oxygen-containing gas, such as air for example, isemployed as the milling fluid. This oxidizing tendency in the mill,however, is lessened somewhat by the increased inhibiting affect of thevolatilized portion of the residual oils.

The following examples are provided for further illustration of theeffectiveness of the method comprising the present invention for coatingsubstantially oil-free molybdenum disulfide powder and the oxidationstability characteristics of the resultant micronized powder blendrelative to a typical oil-free powder. It will be understood that theexamples are provided by way of illustration and are not intended to belimiting in any way:

EXAMPLE I A 1,000 lb. quantity of a dry retorted molybdenum disulfidepowder containing .0 2% oil and having particle sizes predominantly in arange of from about to about 250 microns was charged to a fluid energyimpact pulverization mill employing air at room temperature and wasmicronized to an average particle size in the order of about 1 micron orless. The resultant micronized powder had an oil content of 0.02%.

EXAMPLE II A 1,000 lb. lot of the dry retorted molybdenum disulfidepowder feed material of Example I was blended with 23.5 lbs. ofsubstantially dry molybdenum disulfide powder containing 6.7% oil in ablender until a substantially uniform powder blend was obtained. Theblended powder mass was then charged to a fluid energy impactpulverization mill employing air at room temperature and micronized toan average particle size in the order of about 1 micron or less. Theresultant micronized powder blend had a residuary oil content of 0.18%.

EXAMPLE III A 1,000 lb. lot of the dry retorted powder feed material ofExample I was blended with 40 lbs. of a substantially dry molybdenumdisulfide powder containing 6.7% oil in a blender until substantiallyuniform mixture was obtained. The powder blend was thereafter charged toa fluid energy impact pulverization mill employing air at roomtemperature and micronized to an average particle size in the order ofabout 1 micron or less. The resultant micronized powder contained 28%oil.

EXAMPLE iv A blend was prepared similar to that of Example III, whichwas charged to a fluid energy impact pulverization mill employing airheated to a temperature of about 195 F. and micronized to an averageparticle size in the order of about 1 micron or less. The resultantpulverized powdered blend had a residuary oil content of about .23%.

The oxidation stability characteristics of each of the powders ofExamples I-IV Were tested by placing a portion of each sample in aclosed container in which the samples had access to air at roomtemperature with normal or low humidity to simulate typical storage andshipping conditions. The samples were periodically analyzed for theirmolybdenum oxide contents. In addition, a portion of each of the samplesof Examples I-IV were spread in an open pan and were exposed to air atroom temperature providing accelerated oxidation test in which the 6samples were subjected to greater humidity variations. The samples wereperiodically analyzed for their molybdenum oxide contents. The data thusobtained on the powders of Examples I-IV are tabulated below:

Oxidation Stability Characteristics SAMPLES STORED IN CLOSED CONTAINERSM003 Content, percent:

Before milling I. O. 02 0.02 0.02 0.02 After 10 days 0. 054 0. 067 O.045 0. 045 After 32 days 0. 104 0. 075 0. 067 0. 067 After days 0. 1560. 093 0. 101 0. 087

SAMPLES STORED IN OPEN PANS M00; Content, percent Before Milling- 0.020.02 0.02 0. 02 After 10 Days 0. 112 0. 064 0. 059 0. 063 After 32 Days0. 142 0. 085 0. 070 0. 066 After 120 Days 0.201 0. 154 0. 120 0. 095

As will be noted from the data obtained on the test samples, thesubstantially oil-free molybdenum disulfide powder represented byExample I increased sharply in percentage of M00 after the impactpulverization thereof and continued to increase in percentage of M00 onaging while stored in the closed test containers. On the other hand, thepercentage of M00 in the pulverized powder blends represented byExamples II-IV increased at a slower rate the higher the oil content.

Additional samples of mixtures substantially oil-free molybdenumdisulfide powder and small amounts of molybdenum disulfide powdercontaining oil were micronized. In order to determine the increase inoxidation in terms of acidity, shelf life tests were run as follows:

1 M1. N/lO, KOH/lO gm. Sample.

The trend of increase in oxidation as shown by the increase in AcidNumber is shown in the above table. The acidity increases with time.

In accordance with the method hereinbefore described, oil-free orsubstantially oil-free (containing less than 0.03% oil) molybdenumdisulfide powders can be substantially uniformly coated with controlledquantity of oily substance by blending the oil-free powder with a powdercontaining a known residual oil content of up to about 8% oil providinga blend containing from 0.05% up to about 8% oil, preferably from about0.15% to 6% oil, and impact pulverizing the powder blend in a fluidenergy mill producing a pulverized powder product that contains from0.05 up to about 8% oil, and preferably from 0.05 to about 6% oil. Inaddition, the method also enables the concurrent removal of any waterfrom the feed material by controlling the milling conditions wherebywater contents ranging up to about 16% are reduced to a level belowabout 0.5% in the pulverized product.

While it Will be apparent that the preferred embodiments hereinillustrated are well calculated to fulfill the objects above stated, itwill be appreciated that the invention is susceptible to modification,variation and change without departing from the proper scope or fairmeaning of the subjoined claims.

What is claimed is:

1. The method of preparing a molybdenum disulfide powder comprising thesteps of providing a blended powdered mass of molybdenum disulfidecontaining from 0.05% to about 8% oil comprising a mixture ofoil-contaim'ng molybdenum disulfide powder and a substantially oil-freemolybdenum disulfide powder, and thereafter pulverizing said blendedpowdered mass in a fluid energy impact pulverization mill therebyconcurrently comminuting said blended powdered mass and coating theparticles of said oil-free powder with oil supplied from saidoil-containing powder producing a substantially uniformly oil coatedpulverized powdered product containing at least 0.05% oil.

2. The method of preparing a molybdenum disulfide powder comprising thesteps of blending a substantially oil-free molybdenum disulfide powderwith an oil-containing molybdenum disulfide powder in proportions so asto provide a powder blend containing an oil content ranging from 0.05%up to about 8%, pulverizing said powder blend in a fluid energy impactpulverization mill thereby concurrently comminuting said powder blendand coating the particles of said oil-free powder with oil supplied fromsaid oil-containing powder and producing a substantially uniformly oilcoated pulverized powdered product containing at least 0.05 oil.

3. The method of preparing a molybdenum disulfide powder comprising thesteps of blending a wet, oil-containing molybdenum disulfide powder witha substantially oil-free molybdenum disulfide powder in proportions soas to provide a powder blend having an oil content ranging from 0.05% upto about 8% and containing up to about 16% water, and thereafterpulverizing said powder blend in a fluid energy impact pulverizationmill thereby concurrently comminuting said powder blend and coating theparticles of said oil-free powder with oil supplied from saidoil-containing powder and reducing the water content to a level of lessthan about .5% by the action of said mill producing a substantially dryand uniformly oil coated pulverized powdered product containing at least0.05% oil.

4. The method of preparing a molybdenum disulfide powder comprising thesteps of blending a molybdenum disulfide powder containing up to about8% oil with a substantially oil-free molybdenum disulfide powder inproportions so as to provide a powder blend having an oil content fromabout .15 up to about 8%, and thereafter pulverizing said powder blendin a fluid energy impact pulverization mill thereby concurrentlycomminuting said powder blend and coating the particles of said oil-freepowder with oil supplied from said oil-containing powder producing asubstantially uniformly oil coated pulverized powdered productcontaining at least 0.05 oil.

5. The method as defined in claim 4, further characterized by the factthat said powder blend contains up to about 16% water which is reducedto a level less than about .5% by controlling the volume and temperatureof the gaseous fluid to said mill and the rate of feed of said powderblend to said mill.

6. The method of preparing a molybdenum disulfide powder comprising thesteps of blending a molybdenum disulfide powder containing up to about8% oil with a substantially oil-free molybdenum disulfide powder inproportions so as to provide a powder blend having an oil contentranging from 0.05 to about .5 and thereafter pulverizing said powderblend in a fluid energy impact pulverization mill thereby concurrentlycomminuting said powder blend and coating the particles of said oil-freepowder with oil supplied from said oil-containing powder producing asubstantially uniformly oil coated pulverized powdered productcontaining from 0.05 to about .5% oil.

7. The method as defined in claim 6, further characterized by the factthat said blended powdered mass contains up to about 16% Water which isreduced to a level less than about .5 by controlling the volume and thetemperature of the gaseous fluid supplied to said mill and the rate offeed of said powder blend to said mill.

8. The method of preparing a molybdenum disulfide powder comprising thesteps of blending a molybdenum disulfide powder containing up to about8% oil with a substantially oil-free molybdenum disulfide powder in theproportions so as to provide a powder blend having an oil contentranging from about .15 to about .5 and thereafter pulverizing saidpowder blend in a fluid energy impact pulverization mill therebyconcurrently comminuting said powder blend and coating the particles ofsaid oilfree powder with oil supplied from said oil-containing powderproducing a substantially uniformly oil coated pulverized powderedproduct containing from 0.05 to about .5% oil.

9. The method as defined in claim 8, further characterized by the factthat said powder blend contains up to about 16% water which is reducedto a level less than about .5 by controlling the volume and temperatureof the gaseous fluid supplied to said mill and the rate of feed of saidpowder blend to said mill.

10. The method of preparing a molybdenum disulfide powder comprising thesteps of blending a flotation extraction refined molybdenum disulfidepowder containing from about 1% to about 8% flotation oil with asubstantially oil-free molybdenum disulfide powder containing less thanabout 0.03% oil in the proportions so as to provide a powder blendhaving an oil content ranging from 0.05% up to about 6%, and thereafterpulverizing said powder blend in a fluid energy impact pulverizationmill thereby concurrently comminuting said powder blend and coatingtheparticles of said oil-free powder with oil supplied from saidoil-containing powder producing a substantially uniformly oil coatedpulverized powdered product containing from 0.05 to about 6% oil.

11. The method as defined in claim 10, further characterized by the factthat said powder blend contains up to about 16% water which is reducedto a level of less than about .5 by controlling the volume andtemperature of the gaseous fluid supplied to said mill and the feed rateof said powder blend to said mill.

References Cited in the file of this patent UNITED STATES PATENTS2,686,156 Arntzen et a1. Aug. 10, 1954

1. THE METHOD OF PREPARING A MOLYBDENUM DISULFIDE POWDER COMPRISING THESTEPS OF PROVIDING A BLENDED POWDERED MASS OF MOLYBDENUM DISULFIDECONTAINING FROM 0.05% TO ABOUT 8% OIL COMPRISING A MIXTURE OFOIL-CONTAINING MOLYBDENUM DISULFIDE POWDER AND A SUBSTANTIALLY OIL-FREEMOLYBDENUM DISULFIDE POWDER, AND THEREAFTER PULVERZING SAID BLENDEDPOWDERED MASS IN A FLUID ENERGY IMPACT PULVERIZATION MILL THEREBYCONCURRENTLY COMMINUTING SAID BLENDED POWDERED MASS AND COATING THEPARTICLES OF SAID OIL-FREE POWDER WITH OIL SUPPLIED FROM SAIDOIL-CONTAINING POWDER PRODUCING A SUBSTANTIALLY UNIFORMLY OIL COATEDPULVERIZED POWDERED PRODUCT CONTAINING AT LEAST 0.05% OIL.